Circumferential antenna for an implantable medical device

ABSTRACT

An apparatus and method for enabling far-field radio-frequency communications with an implantable medical device in which an antenna is embedded within a dielectric around the periphery of the device. Such a circumferential antenna saves space while still permitting far-field telemetry over a desired range of frequencies.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/921,653, filed on Aug. 3, 2001, the specification of whichis incorporated by reference herein.

FIELD OF THE INVENTION

[0002] This invention pertains to implantable medical devices such ascardiac pacemakers and implantable cardioverter/defibrillators. Inparticular, the invention relates to an apparatus and method forenabling radio-frequency telemetry in such devices.

BACKGROUND

[0003] Implantable medical devices, including cardiac rhythm managementdevices such as pacemakers and implantable cardioverter/defibrillators,usually have the capability to communicate data with a device called anexternal programmer via a radio-frequency telemetry link. A clinicianmay use such an external programmer to program the operating parametersof an implanted medical device. For example, the pacing mode and otheroperating characteristics of a pacemaker may be modified afterimplantation in this manner. Modem implantable devices also include thecapability for bidirectional communication so that information can betransmitted to the programmer from the implanted device. Among the datawhich may be telemetered from an implantable device are variousoperating parameters and physiological data, the latter either collectedin real-time or stored from previous monitoring operations.

[0004] Telemetry systems for implantable medical devices utilizeradio-frequency energy to enable bidirectional communication between theimplantable device and an external programmer. An exemplary telemetrysystem for an external programmer and a cardiac pacemaker is describedin U.S. Pat. No. 4,562,841, issued to Brockway et al. and assigned toCardiac Pacemakers, Inc., the disclosure of which is incorporated hereinby reference. A radio-frequency carrier is modulated with digitalinformation by, for example, amplitude shift keying where the presenceor absence of pulses in the signal constitute binary symbols or bits.The external programmer transmits and receives the radio signal with anantenna incorporated into a wand which can be positioned in proximity tothe implanted device. The implantable device also generates and receivesthe radio signal by means of an antenna formed by a wire coil wrappedaround the periphery of the inside of the device casing.

[0005] In previous telemetry systems, the implantable device and theexternal programmer communicate by generating and sensing a modulatedelectromagnetic field in the near-field region with the antennas of therespective devices inductively coupled together. The wand must thereforebe in close proximity to the implantable device, typically within a fewinches, in order for communications to take place. This requirement isan inconvenience for a clinician and limits the situations in whichtelemetry can take place.

SUMMARY OF THE INVENTION

[0006] The present invention is an apparatus and method for enablingcommunications with an implantable medical device utilizing far-fieldelectromagnetic radiation. Using far-field radiation allowscommunications over much greater distances than with inductively coupledantennas. Efficient emission and reception of far-field energy in adesirable frequency range, however, requires an antenna structure withcertain minimum dimensions. In accordance with the invention, a wireantenna is embedded in dielectric compartment that wraps around theexterior of the conductive housing of the implantable device in acircumferential orientation. By encapsulating the antenna within thecompartment, the antenna is protected from bending or breakage, does notinterfere with the device at its implanted site, and requires no specialimplantation procedure. The dielectric compartment also separates theantenna from the conductive housing and allows the antenna to functionas a transmission line antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIGS. 1A and 1B illustrate different implementations of anembedded circumferential antenna.

[0008]FIGS. 2A and 2B are cross-sectional views of the circumferentialcompartment.

[0009]FIGS. 3A and 3B illustrate alternative embodiments for connectingthe components of an exemplary cardiac rhythm management device to acircumferential wire antenna.

DETAILED DESCRIPTION

[0010] As noted above, conventional radio-frequency (RF) telemetrysystems used for implantable medical devices such as cardiac pacemakersutilize inductive coupling between the antennas of the implantabledevice and an external programmer in order to transmit and receive RFsignals. Because the induction field produced by a transmitting antennafalls off rapidly with distance, such systems require close proximitybetween the implantable device and a wand antenna of the externalprogrammer in order to work properly, usually on the order of a fewinches. The present invention, on the other hand, is an apparatus andmethod for enabling telemetry with an implantable medical deviceutilizing far-field radiation. Communication using far-field radiationcan take place over much greater distances which makes it moreconvenient to use an external programmer. Also, the increasedcommunication range makes possible other applications of the telemetrysystem such as remote monitoring of patients and communication withother types of external devices.

[0011] A time-varying electrical current flowing in an antenna producesa corresponding electromagnetic field configuration that propagatesthrough space in the form of electromagnetic waves. The total fieldconfiguration produced by an antenna can be decomposed into a far-fieldcomponent, where the magnitudes of the electric and magnetic fields varyinversely with distance from the antenna, and a near-field componentwith field magnitudes varying inversely with higher powers of thedistance. The field configuration in the immediate vicinity of theantenna is primarily due to the near-field component, also known as theinduction field, while the field configuration at greater distances isdue solely to the far-field component, also known as the radiationfield. The near-field is a reactive field in which energy is stored andretrieved but results in no net energy outflow from the antenna unless aload is present in the field, coupled either inductively or capacitivelyto the antenna. The far-field, on the other hand, is a radiating fieldthat carries energy away from the antenna regardless of the presence ofa load in the field. This energy loss appears to a circuit driving theantenna as a resistive impedance which is known as the radiationresistance. If the frequency of the RF energy used to drive an antennais such that the wavelength of electromagnetic waves propagating thereinis much greater than the length of the antenna, a negligible far-fieldcomponent is produced. In order for a substantial portion of the energydelivered to the antenna to be emitted as far-field radiation, thewavelength of the driving signal should not be very much larger than thelength of the antenna.

[0012] An antenna most efficiently radiates energy if the length of theantenna is an integral number of half-wavelengths of the driving signal.A dipole antenna, for example, is a center-driven conductor that has alength equal to half the wavelength of the driving signal. Such a dipoleantenna can be made of two lengths of metal arranged end to end with thecable from a transmitter/receiver feeding each length of the dipole inthe middle. An efficiently radiating resonant structure is formed ifeach length of metal in the dipole is a quarter-wavelength long, so thatthe combined length of the dipole from end to end is a half-wavelength.A shorter antenna can produce a similar field configuration by utilizinga ground plane to reflect electromagnetic waves emitted by the antennaand thereby produce an image field. A monopole antenna is a conductorwith a length equal to one-quarter the wavelength of the driving signalsituated with respect to a reflecting ground plane so that the totalemitted and reflected field configuration resembles that of the dipoleantenna. For implantable medical device applications, carrierfrequencies between 300 MHz and 1 GHz are most desirable. For example,the carrier signal may be selected to be 1 gigahertz, which correspondsto a wavelength in free space of approximately 32 cm in free space. Infree space, a half-wavelength dipole antenna in would optimally beapproximately 16 cm long, and a quarter-wavelength monopole antennawould optimally have a length approximately 8 cm with the housing 101serving as a ground plane. Because the permittivity of body tissues isgreater than that of free space, the corresponding optimum dipole andmonopole antennas in the human body would be approximately half theselengths. If it is desired to use a lower frequency carrier, even longerantennas must be used.

[0013] One way of implementing far-field telemetry in an implantablemedical device is to use an antenna that extends from the devicehousing. The device housing is metallic and forms an electricallyshielded compartment for electronic circuitry that provides particularfunctionality to the device such as cardiac rhythm management,physiological monitoring, drug delivery, or neuromuscular stimulation.The housing also contains circuitry for transmitting and receiving RFcommunications. The antenna could then take the form of a conductorcovered by insulation that extends from the housing and is electricallyconnected to the RF transmitter/receiver within the housing. The antennacould be any conductive structure capable of efficiently radiatingelectromagnetic energy well known to those of skill in the art such as arod, a wire, a patch, or a loop. Wire antennas, however, are simple tomanufacture, and are volumetrically efficient. They also tend to have anear isotropic radiation pattern in the horizontal plane with fewer nulllocations as compared with other types of antennas. This is particularlydesirable with a far-field telemetry system in an implantable devicesince movement of the patient may arbitrarily orient the antenna withrespect to the receiving antenna of the external device.

[0014] An external wire antenna for an implantable medical devicecapable of emitting far-field radiation, however, may require specialimplantation procedures and may also be broken or deformed as a patientmoves, resulting in de-tuning. In accordance with the present invention,therefore, the antenna is embedded in a dielectric and contained withina compartment of the implantable device. An example of a suitabledielectric is the polyurethane resin used in the header portion ofcardiac rhythm management devices where the therapy leads connect to thedevice. The compartment could be, for example, within the device headeritself or a specialized compartment within the housing having adielectric window. For reasons of patient comfort, however, it isdesirable for implanted devices to be as small as possible, and thisconstrains the carrier frequencies that can be used if aquarter-wavelength monopole or half-wavelength dipole antenna is to beembedded in a compartment.

[0015] A particular embodiment of the invention that minimizes spacerequirements but still allows for efficient radiation is a wire antennaembedded in a dielectric compartment that wraps around the exterior ofthe device housing. The wire antenna is thus oriented circumferentiallyaround the periphery of the conductive housing and separated from it bythe insulating dielectric. If the wire is held at a fixed distance fromthe conductive housing by the compartment, the wire exhibits radiationcharacteristics between a transmission line and a monopole antenna. Ifthe wire diameter is small and the separation between the wire andconductive housing is reasonably distant, the wire thus acts as atransmission line antenna. The antenna is thus a one-piece designintegral to the implantable device and permits the antenna to have alonger electrical length. Given the size constraints of implantabledevices, an ideal monopole antenna may not be practical at the desiredcarrier frequency. A lossy transmission line, however, can be made tohave radiation characteristics that resemble the performance of amonopole antenna. Although such a transmission line antenna may not beas efficient as a quarter-wavelength monopole, it does offer a balancedcompromise between size, efficiency, and radiation pattern.

[0016]FIGS. 1A and 1B show different embodiments of an exemplaryimplantable cardiac rhythm management device with a compartmentalizedcircumferential antenna 200 suitable for radiating and receivingfar-field electromagnetic radiation. The device housing 102 is metallicand contains therapy circuitry TC1 for providing particularfunctionality to the device such as cardiac rhythm management,physiological monitoring, drug delivery, or neuromuscular stimulation aswell as circuitry RFC1 for providing RF communications. A battery B1 isused to supply power to the electronic circuitry within the housing. Oneor more therapy leads 310 are connected to the therapy circuitrycontained within the housing by means of a header 103. Each lead 310 isconnected to one or more electrodes 311 adapted for disposition withinor near the heart. The header 103 is a solid block structure made from asynthetic polymer that has feedthroughs therein for routing electricalconnectors between the therapy leads 310 and the therapy circuitry TC1.The antenna compartment 104 is made of dielectric material and extendsfrom the header 103 to wrap circumferentially around a curved portion ofthe device housing 102 with the antenna 200 embedded therein. Theantenna 200 may be constructed of metal wire such as an alloy made ofapproximately 90% platinum and 10% iridium. Such a material is commonlyused for feedthroughs of therapeutic leads and is both mechanicallystrong and biocompatible. This means that no welding or other means ofattachment is required for attaching the antenna to the device and theantenna can be routed from the transmitting and receiving circuitrywithin the housing through the feedthrough to the dielectric compartmentwith no interposing connections required. An alternative antenna andfeedthrough material is niobium, which has a slightly lower resistivitythan the 90% platinum and 10% iridium alloy. In the embodiment shown inFIG. 1A, the wire antenna 200 has a proximal end 200 a that exits thedevice housing through a feedthrough and begins its radiating lengtharound the edge of the device, terminating at the distal end 200 b. Theembodiment of FIG. 1B is similar except that the distal end 200 b of theantenna is shorted to the device housing. FIGS. 2A-B are cross-sectionsof the dielectric compartment 104 from side and end-on views,respectively, that show the mid-line location of the antenna 200 withinthe compartment.

[0017] A transmission line antenna within a dielectric compartment thatis oriented along a surface of the device housing is capacitivelyconnected to the conductive device housing. This includes both thecircumferential antenna shown in FIGS. 1A-B as well as alternate designswhere the antenna and compartment extend linearly along a relativelyflat surface of the device housing. This capacitance along the length ofthe antenna causes losses that lessen the radiation efficiency of theantenna. These losses become larger as the frequency of the drivingsignal increases and as the value of capacitance increases. One way todecrease the value of the capacitance is to increase the distanceseparating the antenna from the device housing, but this necessarilyincreases the size of the device. The preferred separation distancebetween the antenna and the housing will be effected by a number offactors, including the dielectric constant of the compartment materialand the carrier frequency. In an exemplary implementation, however, itmay be desirable that the separation distance between the antenna andhousing be on the order of only 1.5 to 2.5 millimeters. Another way todecrease the capacitance without changing the separation distance is touse a compartment material with a lower dielectric constant. Thematerial commonly used as a header material in pacemakers isthermoplastic urethane (tecothane) which has a dielectric constant ofabout 4.4. If tecothane is used as the compartment material, thecapacitance is then four times greater than would be the case if antennaand housing were separated by air and may result in unacceptable lossesto the antenna. A material with a lower dielectric constant of only 2.1to 2.4 is polytetrafluoroethylene (PTFE). Construction of the antennacompartment with PTFE instead of thermoplastic urethane thus increasesthe radiation efficiency of the antenna by decreasing the capacitancebetween the antenna and the device housing. Absorption of water by thecompartment material also increases its dielectric constant, and PTFE ishydrophobic while tecothane is hydrophilic. Other materials with lowerdielectric constants that could be used as a compartment materialinclude expanded polytetrafluoroethylene (ETFE) with a dielectricconstant of 2.6, and polyetheretherketone (PEEK) with dielectricconstant of 3.6.

[0018] Because of the space requirements of implantable devices, thephysical length of a compartmentalized antenna on the surface of thedevice is constrained. Reducing the capacitive loading of an antenna,however, also decreases its effective electrical length, thuscompromising the ability to operate at lower frequencies. In order toincrease its capacitive loading, the antenna may be coated with a highdielectric material. Coating materials especially suitable for thispurpose are oxides of titanium or aluminum.

[0019] FIGS. 3A-B are block diagrams of an exemplary implantable cardiacrhythm management device showing examples of how a circumferentialantenna may be connected and driven. In the figures, only one therapylead 310 is shown but it should be understood that a cardiac rhythmmanagement device may use two or more such leads. A microprocessorcontroller 302 controls the operation of the therapy circuitry 320,which includes sensing and stimulus generation circuitry that areconnected to electrodes by the therapy leads for control of heartrhythm, and RF drive circuitry 330 for transmitting and receiving acarrier signal at a specified frequency modulated with telemetry data.The conductors of the therapy lead 310 connect to the therapy circuitry320 through a filter 321 that serves to isolate the circuitry 320 fromany RF signals that may be picked up by the lead. The filter 321 may bea low-pass filter or a notch filter such as a choke.

[0020] The microprocessor 302 also outputs and receives the datacontained in the modulated carrier generated or received by the drivecircuitry 330. The RF drive circuitry 330 includes an RF transmitter andreceiver that are connected by a transmit/receive switch 333 to theantenna. The conductor that connects the transmit/receive switch to theantenna passes from the interior of the device housing to the exteriorwhere the antenna is located through a feedthrough 404. An antennatuning circuit may be employed to adjust the impedance of the antenna byloading the antenna with a variable amount of inductance or capacitance.This alters the effective electrical length of the antenna, and henceadjusts its resonance frequency. By matching the antenna impedance tothe impedance of the transmitter/receiver at a specified carrierfrequency, the reactance of the antenna may be tuned out at thatfrequency so that the antenna forms a resonant structure and efficientlytransmits/receives far-field radiation. In FIG. 3A, the antenna 200 isconnected to the transmit/receive switch 333 through a variable tuningcapacitor 402. In FIG. 3B, the antenna is connected to thetransmit/receive switch through a balun transformer 400 in addition tothe tuning capacitor that may allow better impedance matching than whenthe tuning capacitor alone is used. The balun transformer alsoelectrically isolates the internal circuitry from the device housingwhich may be advantageous in some pacemakers and defibrillators wherethe housing or can is utilized as an electrode in delivering pacing ordefibrillation pulses.

[0021] Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

What is claimed is:
 1. A device, comprising: an implantable conductivehousing for containing electronic circuitry; a dielectric compartmentextending along a surface of the housing; a wire antenna embedded withinthe dielectric compartment, wherein the dimensions of the antenna aresuch that a significant portion of radio-frequency energy delivered tothe antenna is emitted as far-field radiation; and, circuitry within thehousing connected to the antenna for transmitting and receiving amodulated radio-frequency carrier.
 2. The device of claim 1 wherein thedielectric compartment extends from a header for the device having afeedthrough therein for routing the wire antenna between thetransmitting and receiving circuitry within the housing and thedielectric compartment.
 3. The device of claim 1 wherein a distal end ofthe antenna is shorted to the device housing.
 4. The device of claim 1wherein the antenna is in a mid-line location within the dielectriccompartment.
 5. The device of claim 1 wherein the antenna within thedielectric compartment is separated from the conductive housing by afixed distance on the order of between 1.5 and 2.5 millimeters.
 6. Thedevice of claim 1 wherein the wire antenna is coated with an oxide oftitanium.
 7. The device of claim 1 wherein the wire antenna is coatedwith an oxide of aluminum.
 8. The device of claim 1 wherein the wireantenna is made of an alloy containing approximately 90% platinum and10% iridium.
 9. The device of claim 1 wherein the wire antenna is madeof niobium.
 10. The device of claim 1 further comprising an antennatuning circuit for matching the impedance of the antenna to thetransmitting/receiving circuitry at a specified frequency of theradio-frequency carrier by loading the antenna with inductance orcapacitance.
 11. The device of claim 10 wherein the tuning circuitcomprises a variable tuning capacitor for adjusting the resonantfrequency of the antenna.
 12. The device of claim 10 wherein the tuningcircuit further comprises a balun transformer.
 13. The device of claim 1wherein the device is a cardiac rhythm management device having rhythmcontrol circuitry electrically connected to one or more electrodesadapted for disposition within or near the heart by one or more therapyleads.
 14. A method for constructing an antenna in an implantabledevice, comprising: extending a dielectric compartment along a surfaceof a conductive housing for containing electronic circuitry; embedding awire antenna within the dielectric compartment, wherein the dimensionsof the antenna are such that a significant portion of radio-frequencyenergy delivered to the antenna is emitted as far-field radiation; and,connecting circuitry within the housing for transmitting and receiving amodulated radio-frequency carrier to the antenna.
 15. The method ofclaim 14 further comprising extending the dielectric compartment from aheader for the device having a feedthrough therein for routing the wireantenna between the transmitting and receiving circuitry within thehousing and the dielectric compartment.
 16. The method of claim 14further comprising shorting a distal end of the antenna to the devicehousing.
 17. The method of claim 14 further comprising embedding theantenna within the dielectric compartment so as to be separated from theconductive housing by a fixed distance on the order of between 1.5 and2.5 millimeters.
 18. The method of claim 14 further comprising coatingthe wire antenna with an oxide of titanium.
 19. The method of claim 14further comprising coating the wire antenna with an oxide of aluminum.20. The method of claim 14 wherein the wire antenna is made of an alloycontaining approximately 90% platinum and 10% iridium.